Image initialization using PNG file

Table of Contents

• Steps for initializing DW image from an image file
  • Step 1: Loading a PNG image file
  • Step 2: Create a DW image file
  • Step 3: Get cuda pointer for image memory
  • Step 4: Initialize DW image using pixel data
• Saving image contents to file
  • Step 1: Get image pixels
  • Step 2: Wait for cuda copy to finish
  • Step 3: Save the pixel bytes to PNG file

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This tutorial demonstrates how to initialize a DW Image. A CUDA image is used as an example to show how it can be initialized using pixels obtained from a PNG file and how color pixels can be read from dw image and saved to PNG file. For interacting with PNG files, publicly available lodepng library is used.

Note

1. Lodepng library mentioned in this tutorial is a 3rd party library for PNG image files. Any other library can also be used for PNG or other file formats to load image file with specific format.

2. Refer to public documentation for Lodepng online for more details, error checking etc.

3. This tutorial uses cuda images. NvMedia or CPU images can also be used similarly by using respective APIs to copy pixels.

4. Error checking macros are for illustration purpose and may not actually be part of DW SDK

Steps for initializing DW image from an image file

Step 1: Loading a PNG image file

As described below, decode the PNG file to get image attributes and pixel data. These pixels will be used to initialize the DW image and the size(wxh) will be used to allocate DW image in next step.

uint8_t* pBytes = nullptr;
uint32_t width        = 0;
uint32_t height       = 0;
uint32_t bitDepth     = 8;
uint32_t decodeRes    = lodepng_decode_file(&pBytes, &width, &height, inFilePath.c_str(), LCT_RGBA, bitDepth);
if (decodeRes != 0)
{
    std::cerr <<": loading png file failed. Error= "<< lodepng_error_text(decodeRes) << "(" << decodeRes << ") \n";
}

Step 2: Create a DW image file

Create an image as described here in "Basic Creation". This will allocate memory for the underlying image allocation.

dwImageProperties props = {};
props.format            = DW_IMAGE_FORMAT_RGBA_UINT8;
props.height            = height;
props.width             = width;
props.memoryLayout      = DW_IMAGE_MEMORY_TYPE_PITCH;
props.type              = DW_IMAGE_CUDA;
CHECK_AND_HANDLE_ERROR(dwImage_create(&image, props, sdkContext));

Step 3: Get cuda pointer for image memory

Get the underlying memory using dwImage_getCUDA API to get the cuda pointer for the underlying image

dwImageCUDA* imageCUDA = nullptr;
CHECK_AND_HANDLE_ERROR(dwImage_getCUDA(&imageCUDA, image));

Step 4: Initialize DW image using pixel data

Using the pixel data obtained above as source, copy it to the cuda pointer using cudaMemcpy2D API as shown below.

size_t elemSize   = 0;
size_t planeCount = 0;
uint32_t planeChannels[DW_MAX_IMAGE_PLANES];
dwVector2ui planeSize[DW_MAX_IMAGE_PLANES];
CHECK_AND_HANDLE_ERROR(dwImage_getDataLayout(&elemSize, &planeCount, planeChannels, planeSize, &imageCUDA->prop));
size_t srcPitch = planeChannels[0] * elemSize * imageCUDA->prop.width;
CHECK_FOR_CUDA_SUCCESS(cudaMemcpy2D(imageCUDA->dptr[0],
                                    imageCUDA->pitch[0], // dst pitch
                                    pBytes,
                                    srcPitch, // src pitch
                                    srcPitch, // src width
                                    imageCUDA->prop.height,
                                    cudaMemcpyHostToDevice),
                       "ImageCreateFromPNG: updateImage: Error: failed to copy to device buffer");

With above steps, a DW image can be initialized with pixel data obtained from png file.

Note

cudaMemcpy2D should be used as source and destination pitch may differ

Appropriate synchronization can be added when using cuda APIs as required

Saving image contents to file

Step 1: Get image pixels

The DW image pixel data is accessible using cuda pointer and can be copied using cudaMemcpy2D with cuda pointer as source and an array as destination. The destination shoulkd be large enough to hold pixel data.

const dwImageProperties& props = imageCUDA->prop;
size_t elemSize   = 0;
size_t planeCount = 0;
uint32_t planeChannels[DW_MAX_IMAGE_PLANES];
dwVector2ui planeSize[DW_MAX_IMAGE_PLANES];
CHECK_AND_HANDLE_ERROR(dwImage_getDataLayout(&elemSize, &planeCount, planeChannels, planeSize, &imageCUDA->prop));
size_t size     = imageCUDA->prop.width * imageCUDA->prop.height * elemSize * planeChannels[0]; // use only 0th plane for planar image
size_t dstWidth = planeChannels[0] * elemSize * imageCUDA->prop.width;
std::unique_ptr<uint8_t[]> bytes(new uint8_t[size]);
CHECK_FOR_CUDA_SUCCESS(cudaMemcpy2D(bytes.get(),
                                    dstWidth,
                                    imageCUDA->dptr[0],
                                    imageCUDA->pitch[0],
                                    dstWidth,
                                    props.height,
                                    cudaMemcpyDeviceToHost),
                       "cudaMemcpy2DA failed");

Note

Before reading image using cudaMemcpy2D, application has to ensure that previous writes has finished.

Step 2: Wait for cuda copy to finish

After cudaMemcpy2D, and before using the destination, application should ensure that copy has finished before using the destination.

Step 3: Save the pixel bytes to PNG file

The pixel data buffer obtained from DW image can now be processed by application as desired or can be saved to an image file. An example to store it as PNG file is shown below.

uint32_t res = lodepng_encode_file(destPath.c_str(), bytes.get(), props.width, props.height, LCT_RGBA, elemSize * 8);
if (res != 0)
{
    std::cerr << " saveImage: lodepng_encode_file returned error= "
                             << lodepng_error_text(res) << "(" << res << ")\n";
}